1. Field of the Invention
This invention relates to a photoelectric sensor.
2. Description of the Related Art
Photoelectric sensors have been conventionally known. A conventional photoelectric sensor involves a mark affixed to a surface of an object. The mark typically has a different color than the surface of the object. The object is conveyed through a detection field and a visible light is emitted from light-emitting device so as to shine (i.e., irradiate, illuminate) onto the mark and the object's surface as they pass through the detection field. A light-receiving device ascertains the reflected light from the mark and the object. The photoelectric sensor is able to detect the mark on the object based upon a quantity of light received. The quantity of light reflected and received varies according to the differences in the color of the mark and the color of the surface of the object. Accordingly, the mark on the object is detected on the basis of a varying quantity of light received.
A quantity of light reflected from the object and then received by the light-receiving device changes according to the relationship between the color of emitted light illuminating the object and the color of the object's surface. More specifically, when one of a red light, green light, and blue light are emitted, the green light is reflected best in a situation where the surface of the object is green. The blue light, which is nearer in the color spectrum to the green light, is reflected better than the red light. The red light has the weakest reflection since the wavelength for red is the farthest away from the green light (of the three colors used). Accordingly, a large quantity of reflected light is desired when an emitted light shines upon a mark in order to accurately detect the mark on the surface of an object with consistency and stability. Conversely, a color of light is desired which exhibits the smallest quantity of reflection when the light is shining on a part of the surface of the object other than the mark. For example, a red light may be irradiated onto an object's surface when the mark is red and the object's surface is green.
In view of the foregoing, JP-A-11-14459 discloses a color discrimination sensor. Quantities of received light are sampled when light-emitting/receiving operations are carried out using red, blue, and green colors, with regard to the mark and the surface of the object other than the mark. For each of the three colors, a difference is obtained between the quantity of received light from the light irradiated onto the mark and the quantity of received light from the light irradiated onto the object's surface. The desired color of the light is the color that exhibits the largest difference between the received (reflected) light obtained by the light-receiving sensors when that color of light is shining on the mark and onto a part of the object's surface other than the mark.
However, the quantity of light reflected and received varies according to the fluttering, consistency, or trim of the objects during the object's movement across a detection area. For example, when an object or the surface of an object is flimsy, such as wrapping film, the quantity of light reflected and received varies according to the undulation of the surface of the object. The difference between the quantities of received light varies with the variations in the quantity of reflected light. Accordingly, the color of light that produces the largest difference between the reflected quantities of light from the mark and the surface of the object is not always the most suitable color for the detection of a mark.